Characteristics of spring maize sap flow and its environmental influencing factors in typical mollisol area

Clarifying the variations of sap flow rate of spring maize in the typical mollisol area and its main control factors is of great significance to explore water consumption from transpiration and regulate field water management. In this study, we installed the wrapped sap flow sensors and TDR probes to continuously monitor the sap flow rate of spring maize during filling-maturity stage and soil water and heat conditions of topsoil. In combination with meteorological data collecting from a nearby automatic weather station, we analyzed the correlation between the sap flow rate of spring maize and environmental factors at different time scales. The results showed that the sap flow rate of spring maize in typical mollisol area had an obvious fluctuation of high diurnal and low nighttime. The instantaneous sap flow rate peaked at daytime, with a value of 139.9 g·h^-1, but being weak at nighttime. The starting time, closing time, and peak values of spring maize sap flow were significantly inhibited in cloudy and rainy days, compared with that in sunny days. On hourly scale, the sap flow rate was significantly correlated to solar radiation, saturated vapor pressure deficit (VPD), relative humidity, air temperature, and wind speed. On daily scale, only solar radiation, VPD, and relative humidity were significantly correlated with sap flow rate, with the absolute values of correlation coefficient being all above 0.7. Due to high soil water content during the observation period, the sap flow rate was not significantly correlated with soil water content and soil temperature of 0-20 cm layer, with the absolute values of correlation coefficient being less than 0.1. Under the condition without water stress, solar radiation, VPD, and relative humidity were the top three influencing factors of sap flow rate in this region, on both hourly scale and daily scale.

[Effects of environment and canopy structure on stem sap flow in a Larix principis-rupprechtii plantation.]

Accurately quantifying the impacts of environmental factors and canopy structure on stem sap flow is of great significance for deeply understanding water use strategies of trees in changing environment. The stem sap flow of Larix principis-rupprechtii plantation was observed using thermal diffusion probes from June to September of 2019 in the Xiangshuihe small watershed of Liupan Mountains, with the meteorological conditions, root-zone soil water content and canopy structure being simultaneously recorded. We first analyzed the relationships of sap flow rate (J_c) to potential evapotranspiration (PET), relative extract water (REW) and canopy leaf area index (LAI), and then quantified their relative contribution to J_c. The results showed that the response of J_c to PET, LAI, and REW conformed to binomial, linearly increase and saturated exponential function, respectively. The J_c model coupling multiple factors was established as a continuous multiplication of the response functions of J_c to PET, REW and LAI, which had good simulation precision. PET was the main factor leading to the difference of J_c in different weather conditions. The average contribution rate of PET had obvious difference in sunny (with a contribution rate of 40.3%), cloudy (4.3%), and rainy days (-26.3%). PET and LAI were the leading factors affecting the J_c variation among months. The ranges of the contribution rates of PET and LAI were from -23.1% to 16.8% and from -12.3% to 11.0%, respectively. The J_c model coupling the multi-factor effect developed in this study could be used to predict J_c, and quantify the impacts of each leading factor, which had the potential to be an effective tool to analyze the water use of trees in the changing environment.

[Responses of photosynthetic physiology and sap flow to the introduction of external dye in Populus ×euramericana cv. '74/76']

The exogenous liquid introduction technology is an effective way to produce the value-added poplar wood with excellent pattern color. This technology was used to add the various concentrated active red dyeing solution (0.2%, 0.4% and 0.6%) into target trees of six-year-old 107 poplar (Populus ×euramericana cv. '74/76'). The photosynthetic gas exchange parameter and sap flow rate were measured by Li-6400 photosynthetic instrument and TDP stem flowmeter, respectively. We analyzed the relationship between photosynthetic parameters, sap flow rate and dye absorption, and the effects of exogenous dye solution on the photosynthetic physiology and sap flow characteristics. The results showed that exogenous dyeing solution significantly inhibited flow rate of poplar trunks. The 0.2% concentrated liquid was far less effective than others (0.4% and 0.6%). The net photosynthetic rate (P_n), stomatal conductance (g_s) and transpiration rate (T_r) of poplars treated with different concentrated dyeing liquids were significantly lower than the control poplar. The intercellular carbon dioxide concentration (C_i) decreased first and then increased. The inhibitory effects of 0.4% and 0.2% concentrated dyeing solutions on photosynthesis were stronger than that of 0.6%. Dye absorption decreased with increasing dye concentration. The maximum liquid flow rate, P_n, g_s and T_r were significantly negatively correlated with the dye content. The contents of chlorophyll (a+b), chlorophyll a and chlorophyll b in exogenous dyeing solution treatments were significantly lower than those of the control at the later stage. The concentration of dyeing solution and introduction time determined the amount of dye absorption. The dye solution 0.4%, which was introduced for three days, could ensure the appropriate dye absorption and reduce the inhibitory effect on the physiological activities of the poplar.

Response of sap flow rate of apple trees to environmental factors in Loess Platea of Western Shanxi Province, China

To clarify the effects of environmental factors on transpiration process of apple trees in rain-fed ecosystem, the dynamics of sap flow in apple trees from Loess Plateau area in western Shanxi Province of China were monitored using the thermal diffusion technique. Meanwhile, environmental factors including meteorological elements and soil moisture content were measured. The results showed that both net solar radiation (R_n) and atmospheric vapor deficit (VPD) were closely correlated with sap flow rate (J_s), as the main ones among all the measured environmental factors. At both hourly and daily scales, the cumulative variances of the three principal components of the environmental factor were above 86%. The first principal component contained VPD and R_n, and explained the variance of over 52% (at hour scale) and 63% (at daily scale), which could be classified as the evaporation demand index (EDI) and as the key comprehensive environmental varia-bles affecting tree sap flow. The second principal component mainly included soil moisture content (SWC) and other factors, which were referred to the soil water and heat supply index. The third principal component mainly included wind speed and other factors, and could be classified into the hydrothermal dissipation index. At the scale of hourly or daily, the response of J_s to EDI showed a significant exponential growth relationship. At the hourly time scale, J_s of apple trees could be accurately modelled based on the first principal component EDI (R²=0.72). At the daily scale, J_s of apple trees could be better modelled based on potential evapotranspiration (ET₀) (R²=0.88). Our results were of great significance for clarifying the responses of water transport in apple tree to environmental factors, estimating water consumption of apple tree based on meteorological factors, as well as directing orchard water management.

[Time lag characteristics of the stem sap flow of Haloxylon ammodendron in the Minqin oasis-desert ectone, China.]

The stem sap flow rate of Haloxylon ammodendron plantation in the Minqin oasis-desert ectone was measured by the thermal dissipation probe (TDP). A cross-correlation analysis was used to estimate the time lag between the stem sap flow and the environmental factors influencing transpiration, including photosynthetically active radiation (PAR) and water vapor pressure deficit (VPD). The results showed that the stem sap flow rate of H. ammodendron had substantial seasonal variation, with the monthly average sap flow being the highest in June and the lowest in August. There was an obvious time lag between the stem sap flow of H. ammodendron and PAR and VPD. The stem sap flow was lagged behind PAR for 80 min but it was ahead of VPD for 114 min. Additionally, the time lag exhibited significant difference among different months during the growth season from May to September. The sap flow of H. ammodendron was more dependent on the variation of PAR on the daily scale than VPD, but it was more closely related to VPD during the day time. The time lag between the stem sap flow and PAR/VPD had no significant correlation with the tree factors (including plant height, ground diameter, diameter at 50 cm height, under branch height, canopy size) and the nocturnal sap flow.